Power system for sterilization systems employing low frequency plasma

ABSTRACT

The present invention provides a sterilization system that includes a low frequency power feedback control system for controllably adjusting a power applied to a low frequency plasma within a vacuum chamber of the sterilization system to remove gas or vapor species from the article. The power has a frequency of from 0 to approximately 200 kHz. The low frequency power feedback control system includes a power monitor adapted to produce a first signal indicative of the power applied to the low frequency plasma within the vacuum chamber. The low frequency power feedback control system further includes a power control module adapted to produce a second signal in response to the first signal from the power monitor, and a power controller adapted to adjust, in response to the second signal, the power applied to the low frequency plasma to maintain a substantially stable average power applied to the low frequency plasma while the article is being processed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to systems and methods for controlling gasdischarge plasmas in sterilization systems that employ gas dischargeplasmas.

2. Description of the Related Art

Plasmas produced using radio frequency (RF) generators in particularhave proven to be valuable tools in processes for the sterilization ofmedical devices. For example, in U.S. Pat. Nos. 4,643,876 and 4,756,882,which are incorporated by reference herein, Jacobs, et al. discloseusing hydrogen peroxide as a precursor in a low temperaturesterilization system that employs RF plasma. The combination of hydrogenperoxide vapor and a RF plasma provides an efficient method ofsterilizing medical devices without using or leaving highly toxicmaterials or forming toxic by-products. Similarly, Jacob, U.S. Pat. No.5,302,343, and Griffiths, et al., U.S. Pat. No. 5,512,244, teach the useof RF plasmas in a sterilization process.

However, there are problems associated with the use of an RF plasma in asterilization process. The RF plasma may leave residual hydrogenperoxide on the sterilized article. The residual amount of hydrogenperoxide remaining on the sterilized article depends upon the RF powerapplied to the article, the amount of time exposed to the RF plasma, andthe material of the article. For example, while some plastics (e.g.,polyurethane) absorb hydrogen peroxide, other materials (e.g., Teflon)absorb relatively little, thereby yielding less residual hydrogenperoxide after sterilization. in addition, inherent inefficiencies inthe energy conversion from the low frequency (e.g., 60 Hz) line voltageto the RF (e.g., approximately 1 MHz-1 GHz) voltage; used to generatethe RF plasma limit the power efficiency of such systems to typicallyless than 50%. Energy efficiency is further reduced by typically 5-20%by virtue of the losses from the required impedance matching networkbetween the RF generator and the load. Such low energy efficiencysignificantly increases the cost per watt applied to the sterilizedarticles. The required instrumentation for using RF electrical energy(e.g., RF generator, impedance matching network, monitoring circuitry)is expensive, which also increases the cost per watt applied to thesterilized articles.

SUMMARY OF THE INVENTION

One aspect of the present invention is a sterilization system thatcomprises a low frequency power feedback control system for controllablyadjusting a power applied to a low frequency plasma within a vacuumchamber of the sterilization system to remove gas or vapor species fromthe article. The power has a frequency of from 0 to approximately 200kHz. The low frequency power feedback control system comprises a powermonitor adapted to produce a first signal indicative of the powerapplied to the low frequency plasma within the vacuum chamber. The lowfrequency power feedback control system further comprises a powercontrol module adapted to produce a second signal in response to thefirst signal from the power monitor, and a power controller adapted toadjust, in response to the second signal, the power applied to the lowfrequency plasma to maintain a substantially stable average powerapplied to the low frequency plasma while the article is beingprocessed.

Another aspect of the present invention is a method of controllablyadjusting a power applied to a low frequency plasma within a vacuumchamber of a sterilization system to remove gas or vapor species fromthe article. The power has a frequency of from 0 to approximately 200kHz. The method comprises monitoring the power applied to the lowfrequency plasma within the vacuum chamber. The method further comprisesgenerating a first signal indicative of the power applied to the lowfrequency plasma. The method further comprises adjusting the powerapplied to the low frequency plasma in response to the first signal tomaintain a substantially stable average power applied to the lowfrequency plasma while the article is being processed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a preferred embodiment of asterilization system compatible with the present invention.

FIG. 2A schematically illustrates a preferred embodiment of acylindrically-shaped electrode with open ends and perforated sides.

FIG. 2B schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and louvered sides.

FIG. 2C schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and solid sides.

FIG. 2D schematically illustrates an alternative embodiment of anelectrode comprising one or more colinear cylindrically-shaped segmentswith open ends and solid sides.

FIG. 2E schematically illustrates an alternative embodiment of anelectrode with a partial cylindrical shape, open ends, and solid sides.

FIG. 2F schematically illustrates an alternative embodiment of acylindrically-symmetric and longitudinally-asymmetric electrode withopen ends and solid sides.

FIG. 2G schematically illustrates an alternative embodiment of one ormore asymmetric electrodes with open ends and solid sides.

FIG. 2H schematically illustrates an alternative embodiment of anelectrode system with a first electrode that is cylindrically-shapedwith open ends and solid sides, and a second electrode comprising a wiresubstantially colinear with the first electrode.

FIG. 2I schematically illustrates an alternative embodiment of agenerally square or rectangular electrode within a generally square orrectangular vacuum chamber.

FIG. 3, which is broken into FIGS. 3a and 3 b, schematically illustratesan embodiment of a low frequency power module compatible with the phaseangle control method of the present invention.

FIG. 4, which is broken into FIGS. 4a and 4 b, schematically illustratesan embodiment of a low frequency power module compatible with theamplitude control method of the present invention.

FIG. 5A schematically illustrates the phase angle control method ofcontrolling the low frequency power applied to the plasma.

FIG. 5B schematically illustrates the amplitude control method ofcontrolling the low frequency power applied to the plasma.

FIG. 6 schematically illustrates a preferred embodiment of a method ofsterilization compatible with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Production of gas discharge plasmas using low frequency (LF) voltagesavoids the various problems inherent in the state of the artsterilization devices and processes which form and use plasmas producedby radio frequency (RF) voltages. First, LF plasma processing leavesless residual reactive species remaining on the sterilized articles thandoes RF plasma processing. Second, generation of the LF plasma is highlyenergy efficient because little or no frequency conversion from the linevoltage is needed. For example, by using no frequency conversion with aline voltage frequency of 60 Hz, the energy efficiency of thesterilization system can reach approximately 85-95%. Use of LF voltagesalso does not require an impedance matching network, thereby avoidingthe associated energy losses. Third, due to the simplifiedinstrumentation and higher energy efficiency of LF generation, the costper watt applied to the sterilized articles using LF plasmas can be aslow as one-tenth the cost per watt of using RF plasmas. Fourth, thesimplified instrumentation used for generating LF plasmas has proven tobe more reliable and robust, and requiring less complicated diagnosticinstrumentation.

FIG. 1 schematically illustrates one preferred embodiment of the presentinvention comprising a sterilization system 10. The sterilization system10 comprises a vacuum chamber 12, a vacuum pump 14, a vacuum pump line15, a vacuum pump valve 16, a reactive agent source 18, a reactive agentline 19, a reactive agent valve 20, a low frequency (LF) power module22, an LF voltage conduit 24, a vent 26, a vent line 27, a vent valve28, a process control module 30, an electrode 32, and a reactive agentmonitor 34. Persons skilled in the art recognize that other embodimentscomprising sterilization systems of different configurations than thatillustrated in FIG. 1 are compatible with the present invention.

In the preferred embodiment of the present invention, the articles (notshown in FIG. 1) to be sterilized are packaged in various commonlyemployed packaging materials used for sterilized products. The preferredmaterials are spunbonded polyethylene packaging material commonlyavailable under the trademark “TYVEK” or composites of “TYVEK” with apolyethylene terephthalate packaging material commonly available underthe trademark “MYLAR”. Other similar packaging materials may also beemployed such as polypropylene. Paper packaging materials may also beused. With paper packaging, longer processing times may be required toachieve sterilization because of possible interactions of the reactiveagent with paper.

The vacuum chamber 12 of the preferred embodiment is sufficientlygas-tight to support a vacuum of approximately less than 40 Pa (0.3Torr). Coupled to the vacuum chamber 12 is a pressure monitor (notshown) which is also coupled to the process control module to provide ameasure of the total pressure within the vacuum chamber. Also coupled tothe vacuum chamber 12 is the reactive agent monitor 34 which is capableof detecting the amount of the reactive agent in the vacuum chamber 12.In the exemplary embodiment of the present invention, the reactive agentis hydrogen peroxide, and the reactive agent monitor 34 measures theabsorption of ultraviolet radiation at a wavelength characteristic ofhydrogen peroxide. Other methods of reactive agent detection compatiblewith the present invention include, but are not limited to, pressuremeasurement, near infrared absorption, and dew point measurement. Thereactive agent monitor 34 is also coupled to the process control module30 to communicate the detected amount of the reactive agent to theprocess control module 30.

In the preferred embodiment of the present invention, inside andelectrically isolated from the vacuum chamber 12 is the electrode 32,which is electrically conductive and perforated to enhance fluidcommunication between the gas and plasma species on each side of theelectrode 32. The electrode 32 of the preferred embodiment generallyconforms to the inner surface of the vacuum chamber 12, spacedapproximately one to two inches from the wall of the vacuum chamber 12,thereby defining a gap region between the vacuum chamber 12 and theelectrode 32. The electrode 32 is coupled to the LF power module 22 viathe LF voltage conduit 24. In the preferred embodiment, with the vacuumchamber 12 connected to electrical ground via a bypass capacitor andshunt resistor, application of an LF voltage between the vacuum chamber12 and the electrode 32 creates an LF electric field which is strongerin a first region 31 which includes the gap region and the vicinity ofthe edges of the electrode 32. The LF electric field is weaker in asecond region 33 where the sterilized articles are placed. Generally, inother embodiments, the LF electric field can be generated by applying anLF voltage between the electrode 32 and a second electrode in the vacuumchamber 12. In such embodiments, the first region 31 includes the gapregion between the two electrodes, and the vicinity of the edges of oneor both of the electrodes. The preferred embodiment in which the vacuumchamber 12 serves as the second electrode is one of the many differentways to generate the gas plasma.

In the preferred embodiment illustrated in FIG. 2A, acylindrically-shaped electrode 32 provides fluid communication betweenthe gas and plasma on each side of the electrode 32 through the openends of the electrode 32 as well as through the perforations in the sideof the electrode 32. These open ends and perforations permit gaseous andplasma species to freely travel between the first region 31 between theelectrode 32 and the walls of the vacuum chamber 12 and the secondregion 33 where the sterilized articles are placed. Similarly, asillustrated in FIGS. 2B-2I, other configurations of the electrode 32provide fluid communication between the first region 31 and the secondregion 33. FIG. 2B schematically illustrates a cylindrically-shapedelectrode 32 with open ends and louvered openings along its sides. FIG.2C schematically illustrates a cylindrically-shaped electrode 32 withopen ends and solid sides. FIG. 2D schematically illustrates anelectrode 32 comprising a series of colinear cylindrically-shapedsegments with open ends and solid sides. FIG. 2E schematicallyillustrates an electrode 32 with a partial cylindrical shape, open endsand solid sides. FIG. 2F schematically illustrates acylindrically-symmetric and longitudinally-asymmetric electrode 32 withopen ends and solid sides. FIG. 2G schematically illustrates anasymmetric electrode 32 with open ends and solid sides. More than oneelectrode can be used to generate the plasma. FIG. 2H schematicallyillustrates an electrode system with a first electrode 32 that iscylindrically-shaped with open ends and solid sides, and a secondelectrode 32′ comprising a wire substantially colinear with the firstelectrode 32. The LF voltage is applied between the first electrode 32and the second electrode 32′. In this embodiment, the first region 31 isthe region between the first electrode 32 and the second electrode 32′,and the second region 33 is between the first electrode 32 and thevacuum chamber 12. FIG. 2I schematically illustrates a generally squareor rectangular electrode within a generally square or rectangular vacuumchamber. The various configurations for generally cylindrical electrodesschematically illustrated in FIGS. 2A-2H can also be applied to thegenerally square or rectangular electrode of FIG. 2I. Each of theseembodiments of the electrode 32 provide fluid communication between thefirst region 31 and the second region 33.

The vacuum pump 14 of the preferred embodiment is coupled to the vacuumchamber 12 via the vacuum pump line 15 and the vacuum valve 16. Both thevacuum pump 14. and the vacuum pump valve 16 are coupled to, andcontrolled by, the process control module 30. By opening the vacuumvalve 16, gases within the vacuum chamber 12 are pumped out of thevacuum chamber 12 through the vacuum pump line 15 by the vacuum pump 14.In certain embodiments, the vacuum valve 16 is capable of being openedto variable degrees to adjust and control the pressure in the vacuumchamber 12.

The reactive agent source 18 of the preferred embodiment is a source offluid coupled to the vacuum chamber 12 via the reactive agent line 19and the reactive agent valve 20 The reactive agent valve 20 is coupledto, and controlled by, the process control module 30. The reactive agentsource 18 of the preferred embodiment comprises reactive agent species.In the preferred embodiment, the reactive agent species comprises agermicide which is a sterilant or a disinfectant, such as hydrogenperoxide. In addition, the germicide supplied by the reactive agentsource 18 can be in gas or vapor form. By opening the reactive agentvalve 20, reactive agent atoms and molecules from the reactive agentsource 18 can be transported into the vacuum chamber 12 via the reactiveagent line 19. In certain embodiments, the reactive agent valve 20 iscapable of being opened to variable degrees to adjust the pressure ofthe reactive agent in the vacuum chamber 12. In the exemplary embodimentof the present invention, the reactive agent species of the reactiveagent source 18 comprising hydrogen peroxide molecules.

The vent 26 of the preferred embodiment is coupled to the vacuum chamber12 via the vent line 27 and the vent valve 28. The vent valve 28 iscoupled to, and controlled by, the process control module 30. By openingthe vent valve 28, vent gas is vented into the vacuum chamber 12 via thevent line 27. In certain embodiments, the vent valve 28 is capable ofbeing opened to variable degrees to adjust the pressure of the air inthe vacuum chamber 12. In the exemplary embodiment of the presentinvention, the vent 26 is a High Efficiency Particulate-filtered Air(HEPA) vent which provides filtered: air as the vent gas. Other ventgases compatible with the present invention include, but are not limitedto, dry nitrogen, and argon.

The process control module 30 is coupled to various components of thesterilization system 10 to control the sterilization system 10. In anexemplary embodiment of the present invention, the process controlmodule 30 is a microprocessor configured to provide control signals tothe various other components in response to the various signals receivedfrom other components.

The LF power module 22 of the preferred embodiment is coupled to theelectrode 132 via the LF voltage conduit 24, and is coupled to, andcontrolled by, the process control module 30. The LF power module 22 isadapted to apply a low frequency voltage between the electrode 32 andthe vacuum chamber 12 so as to generate a low frequency plasma in thevacuum chamber 12. FIG. 3, which is broken into FIGS. 3a and 3 b,schematically illustrates an embodiment of the LF power module 22compatible with the phase angle control method of controlling the lowfrequency power applied to the plasma. As illustrated in FIG. 3, the LFpower module 22 comprises an over-power relay 40, a pair of metal oxidevaristors 42, a step-up transformer 50, a flyback current shunt element62, an inductor 64, a capacitor 66, and a LF power feedback controlsystem 70. The LF power feedback control system 70 illustrated in FIG. 3comprises a power controller 60, a current monitor 80, a voltage monitor90, and a power monitor 100 coupled to the current monitor 80 and thevoltage monitor 90. Line voltage (typically 200-240 VAC, 50/60 Hz) isprovided to the step-up transformer 50 via the closed over-power relay40 which is coupled to the LF power feedback control system 70. Forother frequencies, the LF power module 22 may also include a switchingmodule to provide lower frequencies or frequencies up to a few hundredkHz.

In the embodiment illustrated in FIG. 3, the metal oxide varistors(MOVs) 42 are used to suppress transient voltage impulses. Each MOV 42is a multiple-junction solid-state device capable of withstanding largemagnitude impulses with a low amount of let-through voltage. The MOVs 42serve as fast acting “variable resistors” with a low impedance athigher-than-normal voltages and a high impedance at normal voltages.MOVs are manufactured for specific voltage configurations and for avariety of impulse magnitudes. Persons skilled in the art are able toselect MOVs 42 consistent with the present invention.

The output voltage of the step-up transformer 50 is preferably betweenapproximately 100 and 1000 V_(rms), more preferably betweenapproximately 200 and 500 V_(rms), and most preferably betweenapproximately 250 and 450 V_(rms). The output voltage of the step-uptransformer 50 is transmitted to the power controller 60, which providesthe LF voltage to the electrode 32 and vacuum chamber 12 via the flybackcurrent shunt element 62, the inductor 64, the capacitor 66, and the LFpower feedback control system 70. The flyback current shunt element 62provides a path for fly-back current and to tune the circuit, and in thepreferred embodiment the flyback current shunt element 62 is a loadresistor of approximately 1500 ohms. In other embodiments, the flybackcurrent shunt element 62 can be a snubber. The inductance of theinductor 64 is chosen to limit noise spikes in the LF current, and istypically approximately 500 mH. The capacitance of the capacitor 66 ischosen to maximize the efficiency of power transfer to the LF plasma bymatching the resonant frequency of the series LC circuit to thefrequency of the applied LF voltage. For a 60 Hz voltage and aninductance of 500 mH, a capacitance of approximately 13.6 μF providesthe resonant condition for which the impedance of the series LC circuitis approximately zero, thereby maximizing the transmitted LF power.Persons skilled in the art are able to select appropriate values forthese components depending on the frequency of the applied LF voltage ina manner compatible with the present invention.

FIG. 4, which is broken into FIGS. 4a and 4 b, schematically illustratesan embodiment of the LF power module 22 compatible with the amplitudecontrol method of controlling the low frequency power applied to theplasma. As illustrated in FIG. 4, the LF power module 22 comprises anover-power relay 40, a pair of metal oxide varistors 42, a step-uptransformer 55, and a LF power feedback control system 70. The LF powerfeedback control system 70 illustrated in FIG. 4 comprises a highvoltage (HV) DC power supply 51, a voltage-controlled oscillator (VCO)52, a voltage-controlled amplifier (VCA) 53, a HV operational amplifier54, a current monitor 80, a voltage monitor 90, and a power monitor 100coupled to the current monitor 80 and the voltage monitor 90. Linevoltage is provided to the HV DC power supply 51 via the closedover-power relay 40 which is coupled to the LF power feedback controlsystem 70. The output of the HV DC power supply 51 is preferably betweenapproximately 100 and 1000 VDC, more preferably between approximately200 and 500 VDC, and most preferably between approximately 250 and 450VDC.

In the embodiment illustrated in FIG. 4, the VCO 52 generates a sinewaveoutput with a constant amplitude and fixed low frequency from 0 to 1MHz, the low frequency selected by supplying an appropriate set-pointvoltage to the VCO 52. Alternative embodiments can utilize otherwaveforms, e.g., triangular or square waveforms. The LF output of theVCO 52 is supplied to the VCA 53, which serves as a power controller tomaintain a substantially stable average power applied to the lowfrequency plasma. In response to a feedback signal from the powercontrol module 110, the VCA 53 amplifies the LF output of the VCO 52 togenerate an amplified LF voltage with an amplitude between approximately0 and 12 VAC. The amplified LF voltage from the VCA 53 is supplied tothe HV operational amplifier 54 which in response generates a highvoltage LF output with an amplitude determined by the amplitude of theamplified LF voltage from the VCA 53. Appropriate HV operationalamplifiers are commercially available (e.g., Apex Microtechnology,Tuscon, Ariz., part number PA93), and persons skilled in the art areable to select a HV operational amplifier compatible with the presentinvention. Typically, the amplitude of the high voltage LF output fromthe HV operational amplifier 54 is approximately 100 to 150 VAC. Inorder to generate larger amplitude LF voltages to be applied to theplasma, the high voltage LF output from the HV operational amplifier 54can be further amplified by the step-up transformer 55, as illustratedin FIG. 4. Alternatively, the step-up transformer 55 may be omitted ifthe HV operational amplifier 54 is capable of generating a high voltageLF output with the desired amplitude to be applied to the plasma.

In both the phase angle control embodiment illustrated in FIG. 3 and theamplitude control embodiment illustrated in FIG. 4, the LF powerfeedback control system 70 of the LF power module 22 further comprises apower control module 110 coupled to the power monitor 100, which iscoupled to the current monitor 80 and voltage monitor 90. The currentmonitor 80 measures the LF current through the electrode 32 and thevacuum chamber 12. In the preferred embodiment of the present invention,the current monitor 80 includes a current sensor 82 which provides avoltage output indicative of the measured real-time, cycle-by-cycle LFcurrent, a first converter 84 which produces a DC voltage in response tothe RMS of the voltage output of the current sensor 82, and a firstvoltage amplifier 86 which amplifies the DC voltage from the firstconverter 84 to produce a real-time current signal. In addition, thecurrent monitor 80 also includes an over-current detector 88, whichmonitors the DC voltage from the first converter 84 in real-time andsends an error signal to the power control module 110 if the LF currentexceeds a pre-set value, caused for example by a short circuit betweenthe electrode 32 and the vacuum chamber 12. Under such an occurrence,the LF voltage is turned off momentarily. This occurrence can result ina few cycles being lost, however the power is stabilized so that theaverage power is not affected by more than a predetermined tolerance.

The voltage monitor 90 measures the LF voltage between the electrode 32and the vacuum chamber 12. In the preferred embodiment of the presentinvention, the voltage monitor 90 includes a step-down transformer 92which produces a voltage output indicative of the measured real-time,cycle-by-cycle LF voltage, a second converter 94 which produces a DCvoltage in response to the RMS of the voltage output of the step-downtransformer 92, and a second voltage amplifier 96 which amplifies the DCvoltage from the second converter 94 to produce a real-time voltagesignal.

In the preferred embodiment, the power monitor 100 further comprises amultiplier that receives the DC voltages from the current monitor 80 andthe voltage monitor 90, and multiplies these two voltages to produce areal-time power signal proportional to the LF power applied to theplasma between the electrode 32 and the vacuum chamber 12, the real-timepower signal being generated in response to the real-time current andreal-time voltage signals, and transmitted to the power control module110. In other embodiments, the power monitor 100 monitors the powerapplied to the plasma by utilizing a signal indicative of the real-timeimpedance of the plasma with either the real-time current or real-timevoltage signals. In still other embodiments, the power monitor 100monitors the power applied to the plasma by utilizing other real-timesignals which indirectly indicate the power applied to the plasma; e.g.,a real-time signal proportional to the brightness of the glow dischargegenerated by the plasma. Persons skilled in the art can select anappropriate power monitor 100 compatible with the present invention.

The power control module 110 of the preferred embodiment includes afault detector, such as an over-power detector 112 which monitors thereal-time power signal from the, power monitor 100 and opens theover-power relay 40 if the LF power exceeds a pre-set value, therebyextinguishing the LF plasma. After such an occurrence, the control ofrestart can be given to the user or to software. The power controlmodule 110 of the preferred embodiment further comprises an additionalfault detector, such as a thermal switch 114 which detects overheating,and a power control processor 120.

In the preferred embodiment, the power control processor 120 controlsand monitors the status of the LF power feedback control system 70. Thepower control processor 120 is coupled to a user interface 122 whichprovides user input regarding a selected power magnitude setting and aselected power on/off setting. The power control processor 120 is alsocoupled to the power monitor 100, the thermal switch 114, and theover-current detector 88. In the preferred embodiment, the powermagnitude setting can be selected from two power levels, 800 W and 600W. When the power is turned on, the preferred embodiment of the powercontrol processor 120 ensures that a “soft start” condition ismaintained in which the inrush current is minimized. In addition, theuser interface 122 receives signals from the power control processor 120indicative of the status of the sterilization system 10, which iscommunicated to the user.

In the phase angle control embodiment illustrated in FIG. 3, the powercontrol processor 120 is also coupled to the power controller 60. Inthis embodiment, the power control processor 120 transmits a signal tothe power controller 60 in response to signals from the user interface122, power monitor 100, over-current detector 88, and thermal switch 114in order to maintain a substantially stable LF power applied to the LFplasma while avoiding error conditions. In the amplitude controlembodiment illustrated in FIG. 4, the power control processor 120 iscoupled to the VCA 53. In this embodiment, the power control processor120 transmits a signal to the VCA 53 in response to signals from theuser interface 122, power monitor 100, over-current detector 88, andthermal switch 114 in order to maintain a substantially stable LF powerapplied to the LF plasma while avoiding error conditions. In bothembodiments illustrated in FIG. 3 and FIG. 4, the power controlprocessor 120 typically maintains the LF power applied to the LF plasmawithin a tolerance of approximately 0-10% of the specified power level.

Note that not all of the components listed and described in FIG. 3 andFIG. 4 are required to practice the present invention, since FIG. 3 andFIG. 4 merely illustrate particular embodiments of the LF power module22. These components include components for automation, safety,regulatory, efficiency, and convenience purposes. Other embodimentscompatible with the present invention can eliminate some or all of thesecomponents, or can include additional components.

In response to the signal from the power control processor 120, thepower controller 60 of the embodiment illustrated in FIG. 3 controls theLF power applied between the electrode 32 and the vacuum chamber 12 byutilizing phase angle control. Under phase angle control, the duty cycleof the LF power is modified by zeroing the voltage and current appliedbetween the electrode 32 and the vacuum chamber 12 for a portion Δ ofthe cycle period. Such phase angle control is often used to maintainconstant power from electric heaters or furnaces. FIG. 5A schematicallyillustrates the voltage and current for a 100% duty cycle (i.e., Δ=0)and for a reduced duty cycle (i.e., Δ≠0). During normal operations, thepower controller 60 maintains a constant LF power applied to the plasmaby actively adjusting the duty cycle of the LF power in response to thefeedback real-time signal received from the power control module 110 inresponse to the measured LF power. When a fault event is detected by theover-current detector 88 or thermal switch 114, the power controlprocessor 120 reduces the LF power by reducing the duty cycle of the LFpower, and it transmits a signal to the user interface 122 to providenotification of the fault event. Persons skilled in the art are able toselect appropriate circuitry to modify the duty cycle of the LF powerconsistent with the present invention.

Alternatively, the LF power can be controlled by utilizing amplitudecontrol, as in the embodiment illustrated in FIG. 4. Under amplitudecontrol, the LF power is modified by adjusting the amplitude of thevoltage and current applied between the electrode 32 and the vacuumchamber 12. FIG. 5B schematically illustrates the voltage and currentcorresponding to a first LF power setting and a second LF power settingless than the first LF power setting. During normal operations, the VCA53 maintains a constant LF power applied to the plasma by activelyadjusting the amplitude of the LF power in response to the feedbackreal-time signal received from the power control module 110 in responseto the measured LF power. Persons skilled in the art are able to selectappropriate circuitry to modify the amplitude of the LF power consistentwith the resent invention.

The electronics for RF sterilizers are complicated by the need of suchsystems to attempt to closely match the output impedance of the RFgenerator with the plasma impedance at all times in order to maximizepower efficiency and to avoid damage to the RF generator. Plasmaimpedance varies widely during plasma formation, being very high untilthe plasma is fully formed, and very low thereafter. When first ignitinga plasma, the RF generator cannot match the high plasma impedance whichexists prior to the full formation of the plasma, so a large fraction ofthe power output is reflected back to the RF generator. RF generatorshave protection systems which typically limit the RF generator outputduring periods of high reflected power to avoid damage. However, toignite; the plasma, the voltage output of the RF generator must exceedthe threshold voltage required for plasma ignition. The thresholdvoltage is dependent on the chamber pressure, reactive agent, and otheroperating parameters and is approximately 300 _(rms). In an RF system,once ignition has been achieved, and the plasma impedance is therebyreduced, the magnitude of the applied RF voltage must be reduced to asustaining voltage, e.g., approximately 140 V_(rms), to avoid excessivepower delivery. Because the higher RF voltages required for plasmaignition produce excessively high reflected power before full plasmaformation, RF generators require complicated safeguards to preventdamage during the plasma ignition stage.

Conversely, the complexity and rate of ignition failures aresignificantly reduced for LF sterilizers since the LF sterilizers mayoperate using applied voltages above the threshold voltage and have muchless restrictive output impedance matching requirements. During thetimes at which the applied LF voltage equals zero, as seen in FIG. 5A,the LF plasma is extinguished and there is no LF plasma in the vacuumchamber. The LF plasma must then be re-ignited twice each cycle. By onlyoperating in one voltage regime, LF sterilizers have simpler and morereliable electrical systems than do RF sterilizers. These electricalsystems are easier to service and diagnose, thereby reducing the costsassociated with repair. In addition, the higher peak plasma densitiesresulting from LF sterilizers likely result in increased dissociativerecombination on the articles, thereby reducing the amount of residualreactive species remaining on the articles after the sterilizationprocedure.

FIG. 6 schematically illustrates a preferred method of sterilizationusing the apparatus schematically illustrated in FIG. 1. Thesterilization process shown in FIG. 6 is exemplary, and persons skilledin the art recognize that other processes are also compatible with thepresent invention. The preferred process begins by sealing 200 thearticle to be sterilized into the vacuum chamber 12. The vacuum chamberis then evacuated 210 by engaging the vacuum pump 14 and the vacuumvalve 16 under the control of the process control module 30. The vacuumchamber 12 is preferably evacuated to a pressure of less thanapproximately 660 Pa (5 Torr), more preferably between approximately 25to 270 Pa (0.2 to 2 Torr), and most preferably between approximately 40to 200 Pa (0.3 to 1.5 Torr).

In an exemplary process, upon reaching a desired pressure in the vacuumchamber 12, the process control module 30 signals the LF power module 22to energize the electrode 32 within. the vacuum chamber 12. By applyinga LF voltage to the electrode 32, the LF power module 22 ionizes theresidual gases in the vacuum chamber 12, thereby creating 220 a gasdischarge LF plasma inside the vacuum chamber 12. This gas discharge LFplasma is formed from the residual gases in the vacuum chamber 12, whichare primarily air and water vapor. Because this gas discharge LF plasmais created 220 before the reactive agent is injected into the vacuumchamber 12, this gas discharge LF plasma is typically called the“pre-injection”, plasma. The vacuum valve 14 is controllably opened andclosed to maintain a preset vacuum pressure during the pre-injectionplasma step 220. The pre-injection plasma heats the surfaces inside thevacuum chamber 12, including the articles, thereby aiding theevaporation and removal of condensed water and other absorbed gases fromthe vacuum chamber 12 and the articles. A similar pre-injection plasmais described by Spencer, et al. in U.S. Pat. Nos. 5,656,238 and6,060,019, which are incorporated by reference herein. In an exemplaryprocess, the pre-injection plasma is turned off after approximately 0 to60 minutes. Other embodiments that are compatible with the presentinvention do not include the creation of the pre-injection plasma, oruse multiple pre-injection plasmas. In still other embodiments, thevacuum chamber 12 can be vented after the articles are exposed to thepre-injection plasma.

In the preferred process, upon reaching a desired chamber pressure, thevacuum valve 16 is closed, and the reactive agent valve 20 is openedunder the control of the process control module 30, thereby injecting230 reactive agent from the reactive agent source 18 into the vacuumchamber 12 via the reactive agent line 19. In the preferred embodiment,the reactive agent comprises hydrogen peroxide, which is injected in theform of a liquid which is then vaporized. The injected liquid containspreferably from about 3% to 60% by weight of hydrogen peroxide, morepreferably from about 20% to 60% by weight of hydrogen peroxide, andmost preferably from about 40% to 60% by weight of hydrogen peroxide.The concentration of hydrogen peroxide vapor in the vacuum chamber 12may range from 0.125 to 20 mg of hydrogen peroxide per liter of chambervolume. The higher concentrations of hydrogen peroxide will result inshorter sterilization times. Air or inert gas such as argon, helium,nitrogen, neon, or xenon may be added to the chamber with the hydrogenperoxide to maintain the pressure in the vacuum chamber 12 at thedesired level. This injection 230 of reactive agent may occur as one ormore separate injections.

Due to this injection 230 of reactive agent, the chamber pressure of thepreferred process rises to approximately 2000 Pa (15 Torr) or more.After approximately 6 minutes into the injection stage 230, the reactiveagent is permitted to diffuse 240 completely and evenly throughout thevacuum chamber 12. After approximately 1-45 minutes of diffusing 240,the reactive agent is substantially in equilibrium inside the vacuumchamber 12. This diffusing 240 allows the reactive species to diffusethrough the packaging material of the articles, and come into closeproximity, if not contact, with the surfaces of the articles, therebysterilizing the articles. In other embodiments, the diffusion of thereactive agent can be immediately followed by a vent of the vacuumchamber 12.

The vacuum chamber 12 is then partially evacuated 250 by pumping out afraction of the reactive agent from the vacuum chamber 12 bycontrollably opening the vacuum valve 16 under the control of theprocess control module 30. Once the vacuum pressure within the vacuumchamber 12 has reached the desired pressure, the vacuum valve 16 iscontrollably adjusted to maintain the desired pressure, and the processcontrol module 30 signals the LF power module 22 to energize theelectrode 32 within the vacuum chamber 12. In the preferred embodimentin which the reactive agent comprises hydrogen peroxide, the pressure ofthe hydrogen peroxide in the vacuum chamber 12 is preferably less thanapproximately 670 Pa (5 Torr), more preferably between approximately 25and 270 Pa (0.2 to 2 Torr), and most preferably between approximately 40and 200 Pa (0.3 to 1.5 Torr). By applying a LF voltage to the electrode32, the LF power module 22 generates 260 a reactive agent LF plasmainside the vacuum chamber 12 by ionizing the reactive agent. The articleis exposed to the reactive agent LF plasma for a controlled period oftime. In the preferred embodiment, an additional cycle 275 is performed.Other embodiments may omit this additional cycle 275, or may includefurther cycles.

In both RF and LF plasmas, the components of the reactive agent plasmainclude dissociation species of the reactive agent and molecules of thereactive agent in excited electronic or vibrational states. For example,where the reactive agent comprises hydrogen peroxide as in the preferredembodiment, the reactive agent plasma likely includes charged particlessuch as electrons, ions, various free radicals (e.g., OH, O₂H), andneutral particles such as ground state H₂O₂ molecules and excited H₂O₂molecules. Along with the ultraviolet radiation produced in the reactiveagent plasma, these reactive agent species have the potential to killspores and other microorganisms.

Once created, the charged particles of the reactive agent plasma areaccelerated by the electric fields created in the vacuum chamber 12.Because of the fluid communication between the first region 31 and thesecond region 33, some fraction of the charged particles created in thefirst region 31 are accelerated to pass from the first region 31 to thesecond region 33 which contains the articles.

Charged particles passing from the first region 31 to the second region33 have their trajectories and energies affected by the electricpotential differential of the sheath regions between the plasma and thewalls of the vacuum chamber 12 and the electrode 32. The se sheathregions are created by all electron-ion plasmas in contact with materialwalls, due to charged particles impinging from the plasma onto thewalls. Electrons, with their smaller mass and hence greater mobility,are lost from the plasma to the wall before the much heavier and lessmobile ions, thereby creating an excessive negative charge densitysurrounding the walls and a corresponding voltage differential whichequalizes the loss rates of the electrons and the ions. This voltagedifferential, or sheath voltage, accelerates electrons away from thewall surface, and accelerates positive ions toward the wall surface.

The sheath voltage varies for different plasma types, compositions, andmethods of production. For RF plasmas, the sheath voltage is typically40%-80% of the RF voltage applied to the electrode 32. For example, fora root-mean-squared (RMS) RF voltage of 140 V_(rms) applied to theelectrode 32 once the RF plasma is established, the corresponding sheathvoltage is approximately 55-110 V_(rms). An ion entering the sheathregion surrounding the electrode 32 will then be accelerated to anenergy of 55-110 eV. This acceleration of positive ions by the sheathvoltage is the basic principle behind semiconductor processing by RFplasmas.

As described above, for the LF plasmas of the preferred embodiment ofthe present invention, the voltage applied to the electrode 32 may beequal to or greater than the ignition threshold voltage, which istypically 300 V_(rms). In addition, for LF plasmas, the sheath voltageis typically a higher percentage of the applied voltage than for RFplasmas, so the sheath voltage of the preferred embodiment of thepresent invention is then much higher than the sheath voltage for an RFplasma system. This higher sheath voltage thereby accelerates thecharged particles of the LF plasma to much higher energies. Therefore,because the charged particles are accelerated to higher energies, thecharged particles of the LF plasma of the preferred embodiment travelfarther and interact more with the articles than do the chargedparticles of RF plasma sterilizers.

Since the LF electric field changes polarity twice each cycle, thedirection of the electric field acceleration on the charged particlesreverses twice each cycle. For charged particles in the first region 31,this oscillation of the direction of the acceleration results in anoscillation of the position of the charged particles. However, because;of the fluid communication between the first region 31 and the secondregion 33, some fraction of the charged particles are able to pass tothe second region 33 containing the articles from the first region 31before the direction of the electric field acceleration reverses.

The fraction of the charged particles created in the reactive agent LFplasma which enter the second region 33 is a function of the frequencyof the applied electric field. The charged particles have two componentsto their motion—random thermal speed and drift motion due to the appliedelectric field. The thermal speed, measured by the temperature, is thelarger of the two (typically approximately 10⁷-10⁸ cm/sec forelectrons), but it does not cause the charged particles to flow in anyparticular direction. Conversely, the drift speed is directed along theelectric field, resulting in bulk flow of charged particles in thedirection of the applied electric field. The magnitude of the driftspeed is approximately proportional to the magnitude of the appliedelectric field, and inversely proportional to the mass of the chargedparticle. In addition, the magnitude of the drift speed is dependent onthe gas species and chamber pressure. For example, for typical operatingparameters of gas discharge plasma sterilizers, including an averageelectric field magnitude of approximately 1 volt/cm, the drift speed foran electron formed in a gas discharge plasma is typically approximately10⁶ cm/sec.

A charged particle enters the second region 33 containing the articlesonly if it reaches the second region 33 before the polarity of theapplied electric field changes, which would reverse the acceleration ofthe charged particle away from the electrode 32. For example, for anapplied RF electric field with a frequency of 13.56 MHz, the period ofthe electric field is approximately 7.4×10⁻⁸ sec, so an electron onlymoves a distance of approximately 3.7×10⁻³ cm during the half-cycle orhalf-period before the direction of the electric field changes and theelectron is accelerated away from the electrode 32. Due to their muchlarger masses, ions move much less than do electrons. Where the firstregion 31 between the vacuum chamber 12 and the electrode 32 isapproximately 2.54 cm wide, as in the preferred embodiment, only afraction of the charged; particles created by an RF plasma wouldactually reach the second region 33 containing the articles.

Conversely, for an applied LF electric field with a frequency of 60 Hz,the period of the electric field is approximately 16.7×10⁻³ sec, so anelectron can move approximately 8.35×10³ cm before it is acceleratedaway from the electrode 32. Therefore, the use of LF voltages to createthe plasma in the sterilization system 10 of the preferred embodimentresults in more activity in the second region 33, as compared to aplasma generated using RF voltages. This higher activity in LFsterilizers likely contributes to the increased efficiency for theremoval of residual reactive species from the sterilized articles ascompared to RF sterilizers.

The plasma decay time, defined as a characteristic time for the plasmato be neutralized after power is no longer applied, provides anapproximate demarcation between the LF and RF regimes. The plasma decaytime is not known precisely, but it is estimated to be approximately10⁻⁴-10⁻³ sec for the plasma densities used in sterilizer systems, suchas the preferred embodiment of the present invention. This plasma decaytime corresponds to the time a charged particle exists before it isneutralized by a collision with a surface or another plasma constituent,and is dependent on the plasma species generated and the geometries ofthe various components of the sterilization system 10. As describedabove, the LF regime is characterized by a plasma which is extinguishedand re-ignited twice each cycle, i.e., the half-period of the applied LFvoltage is greater than the plasma decay time. Therefore, thesterilization system 10 is continually run at an applied voltage abovethe ignition threshold voltage of the plasma in order to re-ignite theplasma. The estimated approximate range of plasma decay times of10⁻⁴-10⁻³ sec for many of the plasmas compatible with the presentinvention then translates to an upper limit on the low frequency regimeof approximately 1-10 kHz. However, under certain circumstances, higherfrequencies can be tolerated.

Alternatively, the upper limit of the low frequency regime may bedefined as the frequency at which the electron drift speed is too slowfor an electron to traverse the 2.54-cm-wide first region 31 during ahalf-period of the applied LF voltage. Under typical operatinggeometries, this upper limit of the low frequency regime would beapproximately 200 kHz. For other geometries, the upper limit of the lowfrequency regime can be correspondingly different.

In the preferred embodiment of the present invention, the frequency ofthe LF voltage applied to the plasma is preferably from 0 toapproximately 200 kHz, more preferably from 0 to approximately 10 kHz,still more preferably from 0 to approximately 1 kHz, and even morepreferably from 0 to approximately 400 Hz. When selecting the frequencyof the LF voltage applied to the plasma, the frequency is mostpreferably selected to have a half-period greater than the plasma decaytime of the plasma.

In the preferred method, the LF power module 22 remains energized forapproximately 2-15 minutes, during which the plasma removes excessresidual reactive species present on surfaces within the vacuum chamber12, including on the articles. There is a brief rise of the vacuumpressure upon generating 260 the plasma, however, the majority of theresidual removal step 270 is conducted at an approximately constantvacuum pressure of 50 to 70 Pa (0.4 to 0.5 Torr). The residual removalstep 270 is ended by the process control module 30, which turns off theLF power module 22, thereby quenching the plasma.

After the residual removal step 270, the vacuum chamber 12 is vented 280by the process control module 30 which opens the vent valve 28, therebyletting in vent gas from the vent 26 through the vent line 27 and thevent valve 28. In the preferred process,, the vacuum chamber 12 is thenevacuated 290 to a pressure of approximately 40 to 105 Pa (0.3 to 0.8Torr) to remove any remaining reactive agent which may be present in thevacuum chamber 12. The vacuum chamber 12 is then vented again 300 toatmospheric pressure, and the sterilized articles are then removed 310from the vacuum chamber 12.

The LF plasma provides a reduction of the amount of residual reactiveagent molecules remaining on the articles after the sterilizationprocedure is complete. Where the reactive agent comprises hydrogenperoxide, the amount of residual hydrogen peroxide. remaining on thesterilized articles is preferably less than approximately 8000 ppm, morepreferably less than approximately 5000 ppm, and most preferably lessthan approximately 3000 ppm. In a comparison of the amount of residualhydrogen peroxide remaining after a LF plasma sterilization as comparedto a RF plasma sterilization, nine polyurethane test samples wereexposed to hydrogen peroxide during a simulated sterilization cycle inboth a LF sterilizer and a RF sterilizer. Each sample was prepared bywashing with Manuklenz(® and drying prior to sterilization to avoid anycross contamination. The nine samples were then distributed uniformlyacross the top shelf of a standard industrial rack.

A full LF sterilization cycle, which matched nearly exactly theconditions of a standard RF sterilizer cycle, was used to perform thecomparison. The full LF sterilization cycle included a 20-minuteexposure to a pre-injection plasma, a first 6-minute hydrogen peroxideinjection, a vent to atmosphere, a 2-minute diffusion, a first 2-minutepost-injection plasma, a second 6-minute hydrogen peroxide injection, avent to atmosphere, a 2-minute diffusion, a second 2-minutepost-injection plasma, and a vent to atmosphere. Two full LFsterilization cycles were performed and compared to two full RFsterilization cycles. As seen in Table 1, all parameters other than thepost-injection plasma power were maintained as constant as possible fromrun to run.

TABLE 1 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Pre-injection plasma 727 W779 W 751 W 752 W power First post-injection 783 W 874 W 757 W 756 Wplasma power Second post-injection 755 W 893 W 758 W 758 W plasma powerChamber temp. 45° C. 45° C. 45° C. 45° C. nom. nom. nom. nom. Injectionsystem temp. 65-75° C. 65-75° C. 65-75° C. 65-75° C. H₂O₂ concentration17 mg/l 17 mg/l 17 mg/l 17 mg/l Chamber pressure 50 Pa 50 Pa 50 Pa 50 Paduring plasma (0.4 Torr) (0.4 Torr) (0.4 Torr) (0.4 Torr)

Variations in the pre-plasma power were ±3.5%, so the sample temperaturewas approximately constant from run to run. The samples were thenremoved and the residual, analysis was performed.

The LF sterilizer used to generate the LF plasma was operated at 60 Hz,and with an inductor of 500 mH and a capacitor of 13.6 μF. LF plasmapower was determined by multiplying the voltage across the LF plasma bythe current, then averaging on an oscilloscope. The fluctuation level ofthe LF power was approximately 10%. Table 2 illustrates the results ofthe comparison.

TABLE 2 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Average 769 W 884 W 757 W757 W post-injection plasma power H₂O₂ residuals 1973 ± 144 1864 ± 752682 ± 317 2510 ± 203 (ppm)

Exposure to a LF post-injection plasma reduced the residual reactivespecies more effectively than did exposure to a RF post-injection plasmaof comparable power. LF Run 1 had approximately 23% less residualhydrogen peroxide than either RF Run 1 or RF Run 2, even though all hadapproximately the same post-injection plasma power. The LF processestherefore resulted in less residual hydrogen peroxide than did thecorresponding RF process.

The comparison of the two LF sterilization cycles illustrates thatincreased plasma power results in a reduction of the hydrogen peroxideresiduals. Furthermore, the variation between samples, as indicated bythe standard deviation of the residual measurements, was significantlyreduced in the LF process, thereby indicating an increased uniformity ascompared to the RF process.

Although described above in connection with particular embodiments ofthe present invention, it should be understood the descriptions of theembodiments are illustrative of the invention and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A sterilization system that comprises a plasmagenerator and a power feedback control system for controllablymaintaining a predetermined average power value of a power applied to aplasma within a chamber of the sterilization system the power having afrequency of from 0 to approximately 200 kHz, the power feedback controlsystem comprising: a power monitor comprising a current monitor and avoltage monitor; the current monitor adapted to produce a first signalindicative of a current applied to the plasma within the chamber, thevoltage monitor adapted to produce a second signal indicative of avoltage applied to the plasma within the chamber, the power monitoradapted to produce a third signal in response to the first signal andsecond signal, the third signal indicative of the power applied to theplasma within the chamber, a power control module adapted to produce afourth signal in response to the third signal from the power monitor andto a reference signal corresponding to the predetermined average powervalue; and a power controller coupled to the plasma generator andadapted to adjust, in response to the fourth signal, the power appliedto the plasma to maintain the predetermined average power value of thepower applied to the plasma.
 2. The power feedback control system asdescribed in claim 1, wherein the current monitor comprises a currentsensor, a first converter, and a first voltage amplifier.
 3. The powerfeedback control system as described in claim 2, wherein the currentmonitor further comprises an over-current detector coupled to the powercontrol monitor.
 4. The power feedback control system as described inclaim 1, wherein the voltage monitor comprises a step-down transformer,a second converter, and a second voltage amplifier.
 5. The powerfeedback control system as described in claim 1, wherein the powercontrol module comprises a power control processor.
 6. The powerfeedback control system as described in claim 5, wherein the powercontrol processor is coupled to the power controller, the power monitor,and the current monitor.
 7. The power feedback control system asdescribed in claim 5, wherein the power control module further comprisesa fault detector.
 8. The power feedback control system as described inclaim 7, wherein the fault detector is selected from the groupconsisting of an over-power detector and a thermal switch.
 9. The powerfeedback control system as described in claim 1, wherein the powercontrol module is coupled to a user interface adapted to receive userinput and to transmit the user input to the power control module. 10.The power feedback control system as described in claim 1, wherein thepower controller is adapted to adjust a duty cycle of the power appliedto the plasma in response to the fourth signal from the power controlmodule.
 11. The power feedback control system as described in claim 1,wherein the power controller is adapted to adjust an amplitude of thepower applied to the plasma in response to the fourth signal from thepower control module.
 12. The power feedback control system as describedin claim 1, wherein the power has a frequency from 0 to approximately 10kHz.
 13. The power feedback control system as described in claim 1,wherein the power has a frequency from 0 to approximately 400 Hz. 14.The power feedback control system as described in claim 1, wherein thechamber is a vacuum chamber.
 15. A method of controllably maintaining apredetermined average power value of a power applied to a plasma withina chamber of a sterilization system, the power having a frequency offrom 0 to approximately 200 kHz, the method comprising: monitoring acurrent applied to the plasma within the chamber and generating a firstsignal indicative of the current; monitoring a voltage applied to theplasma within the chamber and generating a second signal indicative ofthe voltage; and adjusting the power applied to the plasma in responseto the first signal and the second signal to maintain the predeterminedaverage power value of the power applied to the plasma.
 16. The methodas described in claim 15, wherein adjusting the power applied to theplasma comprises adjusting a duty cycle of the power applied to theplasma.
 17. The method as described in claim 15, wherein adjusting thepower applied to the plasma comprises adjusting an amplitude of thepower applied to the plasma.
 18. The method as described in claim 15,wherein the power has a frequency from 0 to approximately 10 kHz. 19.The method as described in claim 15, wherein the power has a frequencyfrom 0 to approximately 400 Hz.
 20. The method as described in claim 15,wherein the chamber is a vacuum chamber.
 21. A sterilization systemcomprising: a chamber adapted to contain a plasma; a plasma generatorcoupled to the chamber to apply a power to the plasma; and a powerfeedback control system comprising: a power monitor comprising a currentmonitor and a voltage monitor, the current monitor adapted to produce afirst signal indicative of a current applied to a plasma within thechamber, the voltage monitor adapted to produce a second signalindicative of a voltage applied to the plasma within the chamber, thepower monitor adapted to produce a third signal in response to the firstsignal and the second signal, the third signal indicative of the powerapplied to the plasma within the chamber; a power control module adaptedto produce a fourth signal in response to the third signal from thepower monitor and to a reference signal corresponding to a predeterminedpower value; and a power controller coupled to the plasma generator andadapted to adjust, in response to the fourth signal, the power appliedto the plasma to maintain the predetermined power value of the powerapplied to the plasma.